Easter egg: Go to the commentary menu on Disc 1, highlight Off for the second commentary, and hit right, then up. A yellow spider should appear. Hit enter to see a funny introduction with the director.
Easter egg: In Disc 2 of Spider-man 2, go to "Gallery", when your cursor is on "Proceed", hit the up button on your remote, and click on Spider-Man's Spidey sense. You'll go to a clip of Alfred Molina lightening the mood in the studio.
Easter egg: In the Spider-man 2 widescreen Disc 2, under the "Making the Amazing" sub menu, a picture of Doc Ock and his tentacles are in the background. If you click the upper left tentacle (using a DVD-ROM on a computer) a clip of Willem Dafoe pretending to be Doc Ock is shown. When using a normal DVD player, you can use the arrow keys. When "Play All" is selected use the arrow-up key once, the upper left tentacle turns green, then click "Enter" and you will see the same clip.
Answer: While it's never mentioned in the film, the most likely explanation is that Harry, as a rich kid, grew tired of "slumming it" and decided to move back into his mansion.
Harry and Peter's relationship quickly turned sour after Peter refused to discuss Spider-Man as Harry felt Peter knew more than he let on. Peter is Spider-Man's "personal photographer," and Harry only wants to avenge the death of his father. It's not unlikely to assume Harry left to live alone because he was angry Peter wouldn't help him find Spider-Man.